1. Field of the Invention
The present invention generally relates to a method for fabrication devices on semiconductor substrates. More specifically, the invention relates to a method of patterning a layer of magnetic material for fabrication of a magneto-resistive random access memory (MRAM) device.
2. Description of the Related Art
Magneto-resistive random access memory (MRAM) has been developed as a new type of non-volatile memory. Digital information in MRAM is represented by the direction of magnetization of a magnetic material. MRAM has a plurality of memory cells that are interconnected to one another to facilitate storage of information within the MRAM. A memory cell in an MRAM device generally is a multi-layered structure comprising a pair of magnetic layers separated by a tunnel layer. These layers are deposited as overlying blanket films, layer-by-layer, and then featured to form a MRAM device. More specifically, the MRAM device comprises a free (or top) magnetic layer that may change a direction of magnetization and a bottom magnetic layer that has a fixed direction of magnetization. The magnetic layers are separated by a thin tunnel layer formed of a non-magnetic dielectric material such as aluminum oxide (Al2O3) and the like. The top and bottom magnetic layers may each comprise a plurality of layers of magnetic materials such as permalloy (NiFe), cobalt iron (CoFe), and the like. The top and bottom magnetic layers are also supplied with film electrodes (e.g., comprising conductors such as tantalum (Ta), tantalum nitride (TiN), copper (Cu), and the like) to form an electrical connection for the memory cell to the lines of the MRAM.
Fabrication of a MRAM device comprises etch processes in which one or more layers that comprise a film stack are removed, either partially or in total. The MRAM device comprises the layers that are generally formed from materials that may be easily oxidized, sensitive to corrosion or very thin and as such are difficult to etch with no damage to the stack. In the prior art, during etching a MRAM film stack, the etchants may erode the layers of the stack or leave metal-containing residues. These problems arise from low etch selectivity and non-volatile nature of by-products that are formed during an etch process. Such residues may build up along the sides of the film stack and form a conductive veil-like pattern. The conductive residues or eroded layers may cause electrical short-circuits within a MRAM device, e.g., between the magnetic layers separated by the tunnel layer, or may render the MRAM device to operate sub-optimally or not at all.
Therefore, there is a need in the art for a method of patterning magnetic materials for fabrication a magneto-resistive random access memory (MRAM) device.
The present invention is a method of patterning a layer of magnetic material by forming a mask on a film stack that comprises the layer. The mask defines the layer into protected and unprotected regions. The layer is then oxidized to form isolated magnetic regions located at the protected regions.
In one embodiment, the method is used for fabrication of a magneto-resistive random access memory (MRAM) device comprising a MRAM film stack that is formed on a semiconductor substrate. The stack comprises a top electrode layer (e.g., Ta, TaN, and the like), a free magnetic layer (e.g., NiFe, CoFe, and the like), a tunnel layer (e.g., Al2O3 and the like), a magnetic film stack comprising layers of NiFe, Ru, CoFe, PtMn, NiFe, NiFeCr, and the like, a bottom electrode layer (e.g., Ta, TaN, and the like), and a barrier layer (e.g., SiO2 and the like).
The method is a sequence of processes comprising forming a first sacrificial hard mask, plasma etching the top electrode using a chlorine chemistry, performing a first residue removal, oxidizing the free magnetic layer using an oxygen based plasma chemistry, forming a second sacrificial hard mask, plasma etching the magnetic film stack, plasma etching the bottom electrode layer, and performing a second residue removal.